Secondary phase separation vessels/tanks are used to separate an unwanted secondary phases or contaminants such as hydrocarbons from produced water, and generally operate by allowing or facilitating the rising of the unwanted phase(s) or contaminants to the surface of produced water. The unwanted phases or contaminants may then be removed via skimming of the surface of the produced water.
Examples of secondary phase separation vessels/tanks include:
API separators which employ gravity-based separation techniques;
Induced Gas Flotation (IGF) devices, which use injected gas bubbles to aid in separating phases and contaminants; and
Induced Static Flotation (ISF), which likewise use gas bubbles to aid in separating phases and contaminants.
One of the problems with the latter two types of secondary phase separation vessels/tanks is that they do not allow sufficient time, namely time for effective distribution of the gas bubbles within the contaminated fluid and time to allow for such gas bubbles to attach themselves by natural agglomeration to contaminants or unwanted phases to then cause or bring such contaminants or unwanted phases to the surface via flotation for subsequent removal via skimming.
Specifically, in the case of the latter two types of secondary phase separation vessels/tanks, gas bubbles are typically introduced into the center of a chamber via a pipe (referred to as a sparging pipe, and a sparging process), or generated mechanically via motor-driven paddles. Such methods of introducing gas bubbles into the center of the chamber reduces the likelihood of contact of gas bubbles with contaminants which may not be located in the center of the chamber.
Moreover, existing prior art tanks are typically designed to allow contaminants to be floated to the surface of the tank due to differential specific gravities between for example oil and water, and/or to allow agglomeration of gas bubbles to contaminants which causes such contaminants to rise to the surface of the tank. Both techniques then allow for skimming of the contaminants from the surface of the tank and resulting purification of the remaining liquids (leaving the cleanest fluid in the bottom of the tank/vessel). However, both technologies further transfer fluid from the bottom of the chamber (i.e. the cleanest fluid in the chamber being in the bottom of the chamber) when transferring such fluid to a further subsequent chamber for repetition of the process and for subsequent successive purification, in a process that can be termed “bottom to bottom” flow. Problematically with bottom flow, when such fluid is then transferred into a bottom of a subsequent chamber (namely to an area where the cleanest fluid in such subsequent chamber should be) such allows for “short circuiting”-namely allowing such water to again pass from the subsequent chamber to a still further subsequent chamber (i.e., bottom to bottom flow) without sufficient residence time within each chamber to allow removal of impurities therefrom by gas flotation or specific gravity separation.
Also problematic in such “bottom to bottom flow is the so-called “dilution” effect, namely that when injecting cleaned fluid (i.e., the cleanest fluid) from a first chamber into a second successive treatment chamber (where such cleanest fluid from such first chamber is effectively the most contaminated fluid in the second chamber), such fluid is injected into the bottom of the second chamber, where the cleanest fluid typically is situated. This “dilution” effect thereby undoes, to some extent, the phase separation already accomplished, and adds to required residence time to further effect separation.
U.S. Pat. No. 5,766,484 (“the '484 patent”) teaches a tank having an inlet baffle, and providing in FIG. 1 thereof a skimming means 30 and weir means to skim and collect contaminants from the surface, or alternatively simply only a weir to collect contaminants from surface. However, the '484 patent fails to teach apparatus and method which may be easily adapted for successive treatment via a series of chambers, in that it merely teaches a treatment tank having both introduction and removal of fluid from the same side of the tank, which is problematic for use in creating a compact juxtaposed series of chambers for successive treatment of fluids.
Accordingly, improved separation vessels/tanks which avoid the short-circuiting problem and the related “dilution” problem, which better facilitate contaminant-gas bubble contact throughout a fluid to be treated, and which further allow for a compact arrangement of chambers for successive treatment of fluids, are accordingly needed.